On Micro-Pits Produced by the Formation of FeAl₃ in the Rolled Plate of Extra Super Duralumin (7075S-T6)

Abstract

Ashy-black micro-pits were newly found on the surface of rolled plate of extra super duralumin (7075S-T6) by one of the writers. Then, the shape of the pits and the distribution of compositions of Al, Zn, Cu & Fe in the inner surface of them were made clear using an optical microscope, an electron microscope and an X-ray microanalyser. Moreover, it was found that there was no effect of the corrosion by sea-water on the ashy-black micro-pits mentioned above. It was concluded that the ashy-black micro-pits were produced by the formation of FeAl₃ compound in the micro-portion of the rolled plate. The tensile strength of the rolled plate might be reduced by the formation of such FeAl₃ compound in them.